XII Health Consequences of Population Density and Crowding John Cassel The view that crowding and increasing population density are deleterious to health is so widespread and generally accepted as to have become almost a medical axiom. Furthermore, it is currently believed that the harmful effects of crowding not only increase the spread of infectious diseases but also increase the risk of noninfectious disease. Two quotations from a standard text on epidemiology, which with minor variations can be found in all textbooks dealing with the subject, illustrate these opinions. It has long been recognized that crowded communities provide a more fertile ground for the spread of infection than more scattered communities. (1) The deleterious effects of crowding are not, however, confined to matters concerned with the spread of infection, but are also seen in increased mortality from all causes, both infectious and non-infectious. (1) The evidence supporting this point of view is derived largely from four sources: 1. The higher death and morbidity rates that traditionally have been reported from the more densely populated urban centers. 2. The dramatic increase in death rates, primarily due to infectious diseases, that have followed industrialization and urbanism. 3. The higher rates of various diseases reported under crowded conditions such as military training camps, nurseries, etc. 4. Animal studies which have shown that as the number of animals housed together increases-with other factors such as diet, temperature, and sanitation kept constant-maternal and infant mortality rates rise, the incidence of atherosclerosis increases, and the resistance to insults such as drugs, microorganisms, and X rays is reduced. John Cassel is Professor and Chairman of the Department of Epidemiol-onsumer and Food Economics Research Division, 1962.